In a conventional hydraulically operated dockboard, a hydraulic cylinder unit interconnects the frame and the ramp. By operating the cylinder unit to extend the piston rod or ram, the ramp will be pivoted upwardly to an inclined position.
Hydraulic dockboards can also include a separate hydraulic lip lifting cylinder which operates to pivot the lip from a downwardly hanging pendant position to an outwardly extended position. The lip lifting cylinder can be operated through separate manual controls, or in other cases, the lip lifting cylinder is interlinked with operation of the main cylinder so that hydraulic fluid will be supplied to the lip lifting cylinder from the hydraulic system when the ramp is elevated to a predetermined position.
U.S. Pat. No. 4,365,374 is directed to a hydraulic dockboard in which the flow of hydraulic fluid to the lip cylinder is controlled by, and supplied through, the main cylinder. With the construction shown in the aforementioned patent, when the main cylinder is operated to raise the ramp, the piston rod of the cylinder is extended and as the piston approaches the end of its stroke of travel, a passage is opened which supplies hydraulic fluid from the end of the main cylinder to the lip cylinder to thereby operate the lip cylinder and pivot the lip from the pendant position to the extended position.
With the dockboard of the above-mentioned patent, when the ramp is elevated and the flow of pressurized fluid to the main cylinder is terminated, the combined weight of the ramp and the lip will cause the main cylinder to retract and the ramp will pivot downwardly until the extended lip engages the bed of a truck or carrier parked in front of the loading dock. With the lip engaged with the bed of a carrier, the pressure in both the main cylinder the lip cylinder will drop to near ambient pressure.
After the loading operation has been completed, and the truck pulls away from the loading dock, the ramp will again descend by gravity and the hydraulic back pressure is reestablished, preventing the lip from moving to its pendant position, until descent of the ramp has been stopped by engagement of the ramp with a fixed support at a below dock level position. Thus, with the construction shown in U.S. Pat. No. 4,365,374, the lip will not be returned to its downwardly hanging pendant position until the ramp engages a fixed support at a below dock level position.